I have been trying to change my login screen wallpaper to one of my own using ubuntu-tweak. The wallpapers are located in Ubuntu's default wallpaper folder and I have given all users read privileges on them.

During every startup it changes your desktop background on ur_desktop_wallpaper_here.jpg

Note: Whole path to ur_desktop_wallpaper_here may be changed.

The most important thing. Change your desktop background via GUI in Preferences Options on the wallpaper, which you want to have on logon screen.

Restart your computer.

Uncheck startup program 'WallpaperChange'. You don't need it any more now. During the next change of the wallpaper you will use it again.

I had only tested it on ubuntu 13.04 and found a solution by accident. I cannot provide you a technical answer why it works. It may depend on what you had done with your computer before setting logon wallpaper. I will reinstall ubuntu, try this code again and learn if my solution needs any further editing. (Added 4th line in p.1)